1. Field of the Invention
The invention relates to a gas sealing valve for a plastic processing installation and a process to control a plastic processing plant equipped with a gas sealing valve.
2. Description of the Related Technology
In order to produce closed cavity plastic injection moldings it is only necessary to fill a molding tool mold cavity with an amount of plasticized plastic material sufficient to cover the cavity walls to the desired wall thickness. During molding, a gas bubble is formed in a partially filled mold cavity by injection of gas under a pressure of up to 1000 bar and more. The plastic material is injected and pressed on all sides against the walls of the mold cavity. As soon as the plastic material is applied against the walls, the injected gas is returned into a pressure tank in order to conserve and prevent environmental contamination by the gas. The injection molding opening is usually a hollow sprue sealed by introduction of plasticized plastic material following the gas return.
A metering head with a sealing needle valve is applied to the outlet of the mold cavity of the tool for metered injection of the plasticized plastic material. It would be conceivable to provide the valve sealing needle with an axial gas channel connected to the gas tank by an external gas sealing valve for the additional injection of gas. However, if the flow of gas is shut off, plasticized plastic material may penetrate into such a gas channel. This material is thrown into the mold cavity and may lead to formation of internal waviness in the finished injection molding. Furthermore, the axially drilled sealing needle must be replaced with a sealing needle without an axial bore, whenever the injection molding machine is to be used for the production of moldings with a solid fill (i.e. without gas injection).